Loaded 15" BTL Box Design *Pics*

I did 72 in^2 of port on WinISO and got a port length of 36.56, I didn't know exactly how to fit that in, so I took the volume of (24x3x36.56) and adjusted the design to give me the same port volume. I don't know if that design is fundamentally wrong, so if anyone has suggestions to improve it, I would greatly appreciate it.

Here's a pic of the external dimensions:

I wanted the final height to be 20.75" so that the top of the box would be flush with the back seats of my 03' TrailBlazer.

If anyone has any tips/suggestions as to better the design, I really appreciate all the input.